Field of the Disclosure
This disclosure generally relates to expandable members with at least one wave inhibitor and methods of using the same.
Description of the Related Art
Members are installed in openings of workpieces for a variety of reasons, including improving fatigue performance, reinforcement, and installing components. One method of installing a member in the form of a bushing is the FORCEMATE® installation process developed by Fatigue Technology, Inc. The FORCEMATE® installation process is especially suitable for assemblies that will undergo repetitive load cycles and/or may be susceptible to accumulating fatigue damage. Various types of other installation processes can also be used to install bushings.
When a hole is cold expanded or a bushing is installed using the cold expansion method of drawing a tapered mandrel into and through the inner diameter (ID) of the bushing, the majority of material is displaced radially. However, a portion of material is also displaced longitudinally, in-line with the axis of the bushing and just ahead of the mandrel. The amount of material that is displaced longitudinally and the extent to which it is displaced is dependent on many factors including, for example, the thickness of the receiving structure, length of the bushing, the applied expansion, the mandrel taper angle, etc. As the mandrel travels along the length of the hole, the longitudinally displaced material ahead of the mandrel tends to accumulate. As the mandrel nears the exit end of the hole, this material often tends to dissipate in one of two ways. If the cold expansion tooling is configured such that the exit end of the hole or bushing is not constrained, the accumulated material often results in deformation at the exit surface of the bushing or exaggerated extrusion of the bushing. If the cold expansion tooling is configured such that the exit end of the hole or bushing is constrained (e.g., the exit end is in contact with the front of installation tooling), the accumulated material often results in exaggerated radial expansion near or at the exit end of the assembly and may also result in locally increased interference between the mandrel and the hole or the bushing and the receiving structure at the exit end as compared to the interference at the entry end of the assembly. A combination of both radial and axial deformation will often result. Whether resulting in deformation, exaggerated bushing growth, or imbalanced interference from end to end, the longitudinally displaced wave of material associated with a traditional cold expansion or bushing installation may be problematic for several reasons. The wave of material may cause over expansion of the exit side of the workpiece, resulting in unwanted crack initiation, crack growth, shear tears (e.g., shear tears emanating from an edge of the hole in which the bushing is installed), and other failure mechanisms associated with high strains, especially if the expansion exceeds the elongation properties of the workpiece. Accordingly, waves of material formed during expansion processes often decrease fatigue performance, load bearing capabilities, and the like.
When a hole of a workpiece is radially expanded, beneficial residual stresses may be induced in a region of the workpiece about the hole. Increased applied expansion at the exit end of the workpiece, attributable to the wave effect during cold expansion or bushing installation, will often move both a zone of material in compression and a balancing zone of material in tension that is positioned radially away from the edge of the hole. This may result in material in tension proximate to features where tension is undesirable, such as the outer edge of a highly loaded lug.
Installation of a bushing may also produce upset or distorted material at the exit side of the bushing because of the wave of material generating a bi-axial stress distribution in the workpiece along the length of the hole in which the bushing is installed. Bi-axial stress distributions often cause workpiece material at the exit side of the bushing to deform and bulge outwardly and, in some cases, may cause a “volcano” effect. It may be difficult to incorporate a workpiece with bulging surfaces into a load transfer joint because the bulging surfaces may separate faying surfaces in the joint.
If numerous bushings are installed in a parent component, localized deformations in the parent component may produce considerable overall distortion of the parent component. Cupping of regions of the parent component (e.g., an attachment lug or fitting) surrounding the bushings, cupping of bushing flanges (e.g., flanges at exit sides of bushings), or combinations thereof may be the result of a non-uniform stress state through the thicknesses of the parent component and/or bushing. A stress concentration in the workpiece at the exit side of the bushing often results in an outwardly bulging workpiece surface that causes cupping of a bushing flange.
When expanding a bushing into a composite material, a steady increase in applied expansion caused by displaced material building in front of a mandrel and the bi-axial strain induced by the bushing are often detrimental to the surrounding composite material. Non-uniform expansion and bi-axial strain, alone or in combination, often result in localized over expansion, delamination, and damage (e.g., micro-cracking) within the composite sub-structure.
The longitudinal length of a bushing may increase during the installation process such that an exit end of the bushing protrudes outwardly from the workpiece. For example, bushings that have relatively thick walls are well suited for achieving high retention forces. Unfortunately, extremely high axial forces are used to radially expand such thick-walled bushings. These forces, necessary for proper radial expansion, often result in significant lengthening of the bushing, as well as a significant amount of bushing material upset. Additionally, extrusion or growth of the bushing may not be uniform across a non-flanged end of the bushing where, for example, a majority of the growth occurs in a region adjacent to an inner surface of the bushing.